Temperature

A convenient operational definition of temperature is that it is a measure of the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules. The flow of heat is from a high temperature region toward a lower temperature region. The details of the relationship to molecular motion are described in kinetic theory.The temperature defined from kinetic theory is called the kinetic temperature. Temperature is not directly proportional to internal energy since temperature measures only the kinetic energy part of the internal energy, so two objects with the same temperature do not in general have the same internal energy (see water-metal example). Temperatures are measured in one of the three standard temperature scales (Celsius, Kelvin, and Fahrenheit).

Temperature Scales

The triple point of water is 273.16 K, and that is an international standard temperature point. The freezing point of water at one atmosphere pressure, 0.00°C, is 0.01K below that at 273.15 K. If you want to be really precise about it, the boiling point is 373.125 K, or 99.975 °C relative to the standard pressure freezing point. But for general purposes, just 0 °C and 100 °C are precise enough.

Temperature Standard Points

While the typical treatment of temperature scales takes the freezing point of water to be 0C and the boiling point at standard pressure to be 100C, there are more precise treatments of standard points for defining temperatures. By international agreement, one standard point is the triple point of water which has been defined to be 273.16K. The freezing point of water at atmospheric pressure is .01K below this at 273.15K.

In order to obtain a second standard point by means of a thermometer which doesn't depend on the particular substance used to make it, a constant-volume gas thermometer was chosen to measure the boiling point of water. This method is based upon the ideal gas law, i.e., the assumption that if the volume is fixed, the temperature is directly proportional to the pressure. This measurement leads to a boiling point of 373.125K or 99.975 C above freezing at standard pressure. This measurement is independent of the gas used to make the thermometer. Ordinary gases do not behave exactly as ideal gases and are better described by the van der Waals equation of state, but as they are extrapolated to zero pressure, they all project to the same value for the zero of the Kelvin scale.